Experimental investigation of direct initiation of quasi-cylindrical detonations

Initiation of detonation in the cylindrical geometry is important in validating a general theory of initiation, and has particular significance to the interaction of high velocity projectiles with combustible gas. In the present study, an experimental investigation of the direct initiation of gaseous detonations by a line source of condensed explosives (detonating cord) is presented.Due to the high velocity of detonation in the condensed explosive cord (6--7 km/s), the rapid chemical energy release along its length generates a quasi-cylindrical blast wave in the surrounding combustible mixture. For a sufficiently strong detonating cord, the generated blast wave is observed to decay to a stable Chapman-Jouguet (CJ) conical detonation. Below a certain critical energy, the blast wave decays to low velocities without initiating detonation. In the critical regime of initiation, measurements of the combustion front velocity and direct flow visualization indicate that the blast wave decays to sub-CJ velocities before the onset of detonation. The onset of detonation is associated with discrete explosion centers. This phenomenon is similar to what has been previously observed for spherical initiation, suggesting a universal mechanism of detonation initiation.The critical shock radius at which the onset of detonation occurs is correlated to the detonation cell size (lambda) of the mixture, consistently yielding a radius of 4 to 8lambda. Using the invariance of the explosion length R0 in the critical regime of initiation for spherical and cylindrical detonations, it is shown how the critical radii and critical energies scale between the two geometries, clarifying the correct length scales that should be used in the theory of direct initiation of detonation